SINGAPORE - Former president S R Nathan, who suffered a stroke on Sunday (July 31), remains critically ill and unconscious but he is restful and his vital signs are stable, said Prime Minister Lee Hsien Loong.

Mr Lee, who has just returned from Washington, DC, after an official visit to the United States, visited Mr Nathan at Singapore General Hospital on Friday (Aug 5).

Mr Lee said he also chatted with Mrs Nathan and the family.

"This is a difficult time for them," he wrote in a Facebook post on Friday. "The support and encouragement they have received from so many people means a lot to them. They have been touched by the many messages and good wishes."

He added: "Not many people have done as much for Singapore as Mr Nathan has. Please continue to keep him in your thoughts and prayers. "

Minister in the Prime Minister's Office Chan Chun Sing also said on Facebook that he visited Mr Nathan in hospital on Friday. He had been part of Singapore's delegation to Washington, DC.

"Thankful for the doctors and nurses who have been keeping a close watch on Mr Nathan's condition since. Also thankful for the many friends and Singaporeans who have been quietly supporting Mr Nathan's family since," Mr Chan said.

Mr Nathan, 92, is Singapore's sixth and longest-serving president. He was head of state from 1999 to 2011.

Get-well wishes have poured in for him from visitors to SGH, which include senior civil servants and long-time friends like President Tony Tan Keng Yam, chairman of the Council of Presidential Advisers J. Y. Pillay, Deputy Prime Ministers Teo Chee Hean and Tharman Shanmugaratnam.